Google BOM feature: ms since pageload

I expect this feature has been around for quite a while already, but this is the first time I have seen it: stealthy browser object model improvements letting a web page figure out how many milliseconds ago it was loaded. It presumably works in any web browser that is Chrome or that runs the Google Toolbar:

function msSincePageLoad() {
  try {
    var t = null;
    if (window.chrome && chrome.csi)
      t = chrome.csi().pageT;
    if (t === null && window.gtbExternal)
      t = window.gtbExternal.pageT();
    if (t === null && window.external)
      t = window.external.pageT;
  } catch (e) {};
  return t;
}

In Chrome it (chrome.csi().pageT, that is) even reports the time with decimals, for sub-millisecond precision.

Google, this kind of browser improvements should be blogged! Maybe even documented. All I caught in a brief googling for it were two now-garbage-collected tweets by Paul Irish, leading to where it was committed to Chromium, and a screenshot of the feature in action, along with all the other related features not brought up now:



The rest of this post, about how I happened upon it myself, is probably only interesting to the insatiably curious:

Upon having grown weary of all the Chinese automated porn/malware comment spam that passes through Blogger's sub-par spam filtering to my moderation inbox, I decided to replace it with one that is maintained by a service specializing in (and presumably committed to!) blog comments: Disqus. In the process, being lazy, I decided to let their template wizard install itself in my blog template, which required dropping my old blogger template, upgrading it a few versions, and then (only required by my own discerning taste) attempting to manually weed out the worst crud from the new template (none of which was added by Disqus, I might add).

In the apparently uneditable <b:include data='blog' name='all-head-content'/> section, sat a minified version of approximately this code, which seems to look up the vertical position of some latency-testing DOM node passed to it, the first time the visitor scrolls the page, if it's above the fold (which in Blogger's world is apparently a constant 750 pixels into the page :-). And maybe other things.

(function() {
  function Ticker(x) {
    this.t = {};
    this.tick = function tick(name, data, time) {
      time = time ? time : (new Date).getTime();
      this.t[name] = [time, data];
    };
    this.tick("start", null, x);
  }

  window.jstiming = {
    Timer: Ticker,
    load: new Ticker
  };

  try {
    var pt = null;
    if (window.chrome && window.chrome.csi)
      pt = Math.floor(window.chrome.csi().pageT);
    if (pt == null && window.gtbExternal)
      pt = window.gtbExternal.pageT();
    if (pt == null && window.external)
      pt = window.external.pageT;
    if (pt) window.jstiming.pt = pt;
  } catch (e) {};

  window.tickAboveFold = function tickAboveFold(node) {
    var y = 0;
    if (node.offsetParent) {
      do y += node.offsetTop;
      while ((node = node.offsetParent))
    }
    if (y <= 750) window.jstiming.load.tick("aft");
  };

  var alreadyLoggedFirstScroll = false;

  function onScroll() {
    if (!alreadyLoggedFirstScroll) {
      alreadyLoggedFirstScroll = true;
      window.jstiming.load.tick("firstScrollTime");
    }
  }

  if (window.addEventListener)
    window.addEventListener("scroll", onScroll, false);
  else
    window.attachEvent("onscroll", onScroll);
})();